The invention is concerned with an exhaust gas vent tube for upright or stationary cryogenic vessels. The exhaust tube on such double walled insulated vessels for low boiling liquidfied gases serves to channel the gas occurring in the vessel prior to filling and the gas resulting from the filling of the vessel. The tube also serves as overflow drain and is used to limit the maximum fill to 95% of the geometric volume of the vessel. Such an exhaust tube consists, in the interior of the vessel, solely of a vertical pipe open at the bottom. The end of the tube is at the aimed for or desired level of the liquid. During filling, the addition of liquid gases is interrupted as soon as the droplets of liquid come out of the exhaust tube, since these droplets are an indication that the surface of the liquid has reached the end of the exhaust tube.
It was determined by means of weighing that, upon appearance of the first droplets of liquid, the vessel was not, as previously assumed, 95% filled, but short of the maximum fill. This phenomenon is dependent on pressure, that is, the higher the pressure or boiling temperature of the cryogenic liquid in the storage vessel, the greater the difference between maximum and effective fill. The reason for this can be attributed to the drop in pressure occurring during transfer due to sudden vaporization of part of the low boiling gas. Since the fill tube ends at the deepest part in the interior of the cryogenic vessel the gas bubbles which are formed there on account of the drop in pressure and through partial vaporization of the cryogenic liquid, rise through the column of liquid into the gaseous part or headspace. When the bubbles leave the liquid, they pull droplets of liquid with them. At the exit from the exhaust tube into the open, there appears a mixture of gas and liquid. The supply of liquid is thus already stopped prior to the vessel being filled up to maximum weight. The smaller a vessel's diameter, the more this effect is noticeable.